Excitatory amino acids (EAAs) and their receptors play an important role in brain development. It follows that any dysfunction of these neurotransmitters during the perinatal period will have grave consequences, possibly including mental retardation. Kynurenic acid (KYNA), a well-established metabolite of the essential amino acid tryptophan, is present in the brain under physiological conditions and can function as an antagonist of EAA receptors. KYNA, which is preferentially synthesized in glial cells, has therefore been suggested to serve as an endogenous regulator of EAA receptor function. By implication, abnormal cerebral KYNA levels may therefore adversely affect the developing brain. In preliminary studies in rats, it was demonstrated that brain KYNA concentrations can be readily modified both in vitro and in vivo. For example, impairment of cellular energy metabolism and dopamine receptor activation markedly reduce KYNA formation, whereas several 2-oxoacids, i.e. co-substrates of KYNA's biosynthetic enzyme kynurenine amino- transferase, substantially augment KYNA production. An increase in brain KYNA levels can also be achieved pharmacologically by using the specific kynurenine 3-hydroxylase inhibitor PNU 156561A. Interestingly, several of these regulatory mechanisms are age-specific and brain-specific. This project was designed to examine the mechanisms involved in the regulation of brain KYNA using tissue slices, cultured astrocytes and whole animals (rats). Special emphasis will be placed on KYNA modulation in the postnatal period. In the initial set of studies, brain tissue slice from immature and mature rats and cultured human astrocytes will be used in vitro. Changes in the de novo synthesis of KYNA from its bio-precursor kynurenine will be assessed under conditions of energy compromise, 2- oxoacid supplementation and dopamine receptor activation, and the relative importance and possible interdependence of these mechanisms will be recorded. Selected mechanisms, as well as the effects of PNU 156561A, will then be tested in immature rats by in vivo brain microdialysis. Finally, the functional significance of augmentation or reduction in extracellular neurotoxicity in immature animals. Taken together, these studies will comprehensively elaborate KYNA neurobiology in the developing brain and document if and to what extent this endogenous metabolite might be involved in phenomena underlying or associated with mental retardation.